1. Field of the Invention
The present invention relates to the field of face detection and, more particularly, to an image preprocessing method capable of increasing the accuracy of face detection.
2. Description of Related Art
With the advent of computer technologies, real-time face detection and tracking have become an important issue in many applications including video surveillance, teleconferencing, video retrieval, virtual reality, human computer interactions, and so on. It is essential that a face detection algorithm can execute tasks fast and accurately. A known face detection algorithm, which is described in C. C. Han, H. Y Mark, K. C. Yu, and L. H. Chen, “Fast Face Detection via Morphology-Based Pre-Processing,” Pattern Recognition 33, pp. 1701-1712, 2000, and incorporated herein for reference, has shown a very promising performance in both speed and accuracy when dealing with well lit face images. However, the Han's algorithm degrades considerably when images are taken in insufficient or non-uniform lighting conditions.
In the known patents, U.S. Pat. No. 5,012,522 proposes a face decision rule to find a face in an image by examining dark regions. U.S. Pat. No. 5,216,512 proposes a method using an image pick-up device with an image sensor having a rectangular area to locate a face. U.S. Pat. No. 5,596,362 describes a method to find a face candidate by using edge information and then to verify it by fitting with an ellipse. European Patent 0836326 includes a skin tone detector and an eye-nose-mouth region detector to improve the accuracy of the range of a face except fitting with an ellipse. U.S. Pat. No. 5,715,325 proposes a method to identify a face in an image by reducing the resolution, and then fitting a bounding box with the head region. U.S. Pat. No. 5,835,616 describes a method for face detection using templates, which first finds wrinkles and curved shapes of a facial image by blurring filter and edge enhancer, and then confirms the existence of the human face by finding facial features. However all the above results are limited to images with simple background or head-shoulder images and the detected regions are only rough approximations.
U.S. Pat. No. 5,642,431 describes a face detection system including pattern prototypes obtained by a network training process and distance measurement from the applied image to each of the prototypes. Afterwards, Rowley et al. proposed a more complete version for face detection by neural networks. A description of such can be found in H. Rowley, S. Baluja, and T. Kanade, “Neural Network-Based Face Detection,” IEEE Trans. Pattern Analysis and Machine Intelligence, Vol. 20, No. 1, pp. 22-38, January 1998, and is hereby incorporated herein by reference. However the above results are based on the brute force search in the applied image, which is indeed a time-consuming procedure. Therefore it is desired for the above face detection method to be improved.